Cross-correlation of noise and nominal strenth index in burst-prone coal seams

Abstract

To determine the tendency of a coal seam to bunts or other dynamic phenomena -caving, sudden spills of coal, shock bumps -use is made of its seismic activity (noise) [1][2][3][4] and the nominal strength index, determined from the depth of pene~ation of a pointed object into the seam under the effect of a standard force [5,6]. It is assumed that the noise, i.e., the number of fissures arising per unit time during working of the seam reflects the capacity of the seam to undergo brittle fracture. The greater the seismic activity the more readily will the seam fracture and the sooner will it lose stability. Therefore periods of high seismic activity indicate a heightening of the danger of dynamic phenomena. Although very primitive, such a viewpoint is useful because it agrees closely with practical experience: dynamic phenomena are actually manifested during higher seismic activity; this enables one to make a practical seismic prediction of the burst proneness of a seam from variations of the noise [1][2][3][4]. As regards the second parameter (the nominal strength index), it is assumed that it reflects mainly the natural strength of the coal seam: the greater the depth of penetration of the point, the weaker the seam and the more likelihood of the occurrence of dynamic phenomena [5]. This parameter is used for making local forecasts of the likelihood of a burst, the aim being to determine in good time the tendency of coal seams or their individual sectors to exhibit dynamic phenomena [6].It was of interest to determine to what extent both these parameters, which characterize from different angles the strength of a coal seam and its tendency to fracture, depend on rock pressure and to what extent they are mutually correlated. In 1967 a great number of seismic measurements were made by the procedure described in [4] in the steep burst-prone Devyatka seam (sector No. 78, Yunkom colliery, 716 level), accompanied simultaneously by strength measurements with an instrument designed by G. N. Feit [5]. Furthermore, in this sector episodic observations of the pressure curve were made using a hydraulic transducer designed by L. 13. Famin [7]. In the 10 months of these observations the face advance was 400 m.The strength determinations were performed in the lower part of the working-in the cross-hole of the haulage road and in the first and second benches. * The measurement lines were located in the gate ends of the face, in the middle part, and in the foot of the corresponding benches. The measurements were made once per day: in the first and second benches the measurement sites were located at 2 m intervals along the strike of the seam. In the cross-hole of the haulage road the measurements were less regular owing to the nonuniform advance of the latter. Measurements were performed at the surface of the working face; at each measurement point we made five measurements of the depth of penetration and then found the mean value. This Value, l, is one of the correlation parameters.The seismic measurements were made [1-4] with a ...